1. Field of the Invention
This invention is directed to solid electrolytes containing tetrabutyl ammonium thiocyanate and a source of lithium ions and to solid electrolytic cells (batteries) comprising such solid electrolytes. This invention is also directed to methods for improving the coatability of electrolyte solutions prior to curing and for enhancing the cumulative capacity of the solid electrolytic cells by employing a solid electrolyte which contains tetrabutyl ammonium thiocyanate.
2. State of the Art
Electrolytic cells containing a lithium (or lithium based metal) anode, a cathode and a solid, solvent-containing electrolyte are known in the art and are usually referred to as "solid batteries". These cells offer a number of advantages over electrolytic cells containing a liquid electrolyte (i.e., "liquid batteries") including improved safety features. Notwithstanding these advantages, the use of these solid batteries over repeated charge/discharge cycles is substantially impaired because these batteries often quickly lose their charge and discharge capacity after repeated cycles as compared to their initial charge and discharge capacity. Moreover, electrolytes are often difficult to handle in the manufacturing process of the solid batteries.
Solid electrolytic (or electrochemical) cells employ a solid electrolyte interposed between a cathode and an anode. The solid electrolyte contains either an inorganic or an organic matrix as well as a suitable inorganic ion salt. The inorganic matrix may be non-polymeric [e.g, .beta.-alumina, silver oxide, lithium iodide, etc.] or polymeric [e.g., inorganic (polyphosphazine) polymers] whereas the organic matrix is typically polymeric. Suitable organic polymeric matrices are well known in the art and are typically organic polymers obtained by polymerization of a suitable organic monomer as described, for example, in U.S. Pat. No. 4,908,283. Because it is expensive and difficult to form inorganic nonpolymeric matrices into different configurations, they are generally not preferred and the art typically employs a solid electrolyte containing a polymeric matrix. The solid electrolytes also contain an inorganic ion salt and a solvent (plasticizer) which is typically added to the matrix in order to enhance the solubility of the inorganic ion salt in the solid electrolyte and thereby increase the conductivity of the electrolytic cell.
One method of forming a solid, solvent containing electrode is to combine the monomer or partial polymer of the polymeric matrix to be formed with appropriate amounts of the inorganic ion salt and the solvent. This electrolytic mixture is then placed on the surface of a suitable substrate (e.g., the surface of the cathode) and the monomer is polymerized or cured (or the partial polymer is then further polymerized or cured) by conventional techniques (heat, ultraviolet radiation, electron beams, etc.) so as to form the solid, solvent-containing electrolyte. One problem often encountered is that the electrolytic mixture is difficult to coat. This can result in a layer of solid electrolyte that does not have uniform thickness, does not completely cover the substrate, or has thin spots.
When the solid electrolyte is formed on a cathodic surface, an anodic material can then be laminated onto the solid electrolyte to form a solid electrolytic cell. In operation, even though the initial capacity of the cell (or battery) can be relatively high, nevertheless, such solid batteries often exhibit rapid decline in capacity over their cycle life.
The cumulative capacity of a solid battery is the summation of the capacity of a solid battery over each cycle (charge and discharge) in a specified cycle life. Solid batteries having a high initial capacity but which rapidly lose capacity over the cycle life will have low cumulative capacity which interferes with the effectiveness of these batteries for repeated use.
One reason for the capacity loss is that metallic lithium is reactive with a variety of materials (including materials employed, in the solid electrolyte) and thus tends to form "dynamic" passifying films on the surface of the anode. In this regard, such films are termed "dynamic" because they are repeatedly formed and "broken through" during successive cycles. These films must, in fact, be "broken through" with the expenditure of energy in order to allow cycling of the lithium in the system. This reduces the efficiency of the electrochemical cells. In addition to this undesirable use of cell energy, such films are further disadvantageous because they promote the formation of lithium dentrites which can ultimately result in shorting of the electrolytic cell.
In view of the above, the art is searching for methods to enhance the cumulative capacity of such solid electrolytic batteries. In addition, there is a need for electrolytic mixtures with improved coatability.